The Project:
I have several cases where I'm not sure whether or not I should be using a 'Separation of Concerns' approach or a simplistic one. They're all similar to the setup below.

e.g. An Estimate contains multiple Notes

EstimateController.Details -> Displays the details of an Estimate (Name, Description, Etc.) and below that it displays a tabular view of Notes (separated by type).

My Thoughts:

On one hand, it would be a separation of concerns and creating a new controller sounds like the proper way to do things, making use of partial views if necessary.

On the other hand, it seems a bit like overkill to have a controller for all the cases like this. (i.e. Estimates contain: Notes, Specification Numbers, Drawing Numbers, and Addendums. This is similar to multiple other models in the business model as well).

Question:

How trivial is to trivial to justify creating a separate controller for an sub-model?

I've found many places saying that you should have a separate controller for each view model but out of the ones I've seen none of them mention why or mention how trivial of a model is too trivial?